Spectroscopy is often used in the analysis of materials. Typically, the analysis involves transmitting radiation of a range of frequencies through gaseous, liquid, or solid material samples. The materials to be tested include dairy products or engine lubricants to determine contamination, anesthesia gases for monitoring, chemical and pharmaceutical process monitoring, and the like.
Prior art spectroanalytical systems include an entrance aperture defining structure or sample cell for receiving radiation to be analyzed, a dispersion structure or reflective grating for separating the received radiation as a function of wavelength, output optical fibers connected to a detector structure or assembly for detecting specific portions of the dispersed radiation, and a wedge-like optical conditioner structure between the entrance aperture defining structure and the dispersion structure for radiation as a function of wavelength. See U.S. Pat. Nos. 5,856,870 and 6,289,149 incorporated herein by this reference.
In the embodiments disclosed in U.S. Pat. No. 5,856,870, the optical fibers have a high numerical aperture as found in chalcogenide glasses such as arsenic sulfide or arsenic germanium selenide, and heavy metal fluoride glasses such as a mixture of zirconium, barium, lanthanum, and aluminum fluorides, or polycrystalline or single crystal material such a thallium bromoiodide or cesium iodide. The term “numerical aperture” (NA) is the sine of the vertex angle of the largest cone of meridional rays that can enter the system. It is frequently used as a measure of light gathering power.
At intermediate numerical apertures, 0.15-0.35, systems using concave gratings with curved grooves of varying spacing or a concave grating combined with a conical reflective grating or mirror may be used to reduce astigmatism successfully. However, at large numerical apertures, 0.4 to 0.7, which are more compatible with the high numerical aperture optical fibers described above, these approaches fail to deliver the desired efficiency and resolution, especially systems immersed in a low refractive index medium such as a gas.
The prior art design based on an optical “wedge” conditioner enables high numerical aperture (NA>0.7) radiation to be “straightened out” so that it becomes more perpendicular to the grooves of a cylindrical reflection diffraction grating. With a wedge optical conditioner, radiation from the source is increasingly “collimated” or made more parallel to the center plane of the wedge as the wedge angle is initially increased. Thus, the variation in the angle of diffraction caused by radiation oblique to the grating grooves is reduced. The reflected, diffracted radiation is then guided back to the focal surface and the reverse effect restores the radiation to its original high angle condition (high NA), thus preserving the high optical throughput of that system. The prior art system, as well as other known systems, uses point detectors to pick off the diffracted radiation from the focal surface.
While the prior art system including the wedge-like optical conditioner offers many advantages, it does present some limitations as well. Optical fibers and discrete detectors are not always desirable because a system including these elements is “fixed” at the wavelengths where the fibers and detectors are located. With the prior art system described, for each test or analysis that seeks to detect a different chemical substance, for example, the point detectors must be placed in a new arrangement characteristic of the expected wavelengths for that particular chemical substance. Thus, the prior art system lacks versatility in terms of wavelength selection flexibility and the ability to use a large number of detectors.
The use of an array detector would provide additional versatility to the wedge-like optical conditioner structure of the prior art that includes a focal end surface that is curved to approximate the Rowland Circle in order to focus the spatially separated wavelengths onto the fibers leading to the detectors. Use of an array presents a couple of problems. Array detectors are generally available for planar focal surfaces, not for curved focal surfaces. Secondly, array detectors in the infrared are generally thermal detectors, and thermal detector elements cannot contact the conditioner or any thermally conductive filler material between the conditioner and the elements. A thermal detector element in thermal contact with the conditioner would be influenced more by the temperature of the conditioner and less by the infrared radiation emitted from the conditioner.
Furthermore, while in principle it would be possible to design lens or mirror optics to couple the output from the conditioner to, for example, a flat array detector, the optics required to do so would be difficult to fabricate, bulky and expensive.
Known concave grating configurations which produce relatively flat focal fields often require the detector array to be at an appreciable angle with respect to the direction of the diffracted radiation coming from the grating vertex. While this presents little challenge if the optical medium is air, it becomes a problem for high refractive index medium such as zinc selenide (ZnSe), where the rays exiting into gas or vacuum undergo significant refraction and even total internal reflection. FIGS. 1-4 demonstrate the refraction of a converging cone of rays exiting a high index medium (i.e. Zinc Selenide) into a low index medium (i.e. air or vacuum). In each case, the angular distribution of rays is within ±16° of the center ray (the chief ray). It is clear that the largest angle of incidence allowed for the chief ray is less than about 7.5°. In FIG. 1, light rays 12 exit conditioner 14 at exit surface 16 and reach detector plane/focal surface 18 at detector point 20. Angle θ is at 0° where chief ray 24 is perpendicular to exit surface 16 (FIG. 1). FIG. 2 shows chief ray 24 with angle θ at a 5° angle of incidence; FIG. 3 shows chief ray 24 with angle θ at a 7.5° angle of incidence; and FIG. 4 shows chief ray 24 with angle θ at a 10° angle of incidence. Thus, the largest angle of incidence allowed for the chief ray is less than about 7.5° in order for light rays 12 to remain tightly clustered on focal surface 18. At 10° the clustering is significantly degraded as shown in FIG. 4. Therefore, it can be seen that having the exit surface at an appreciable angle with respect to the diffracted radiation coming from the grating vertex presents the undesirable refraction effects shown in FIGS. 2-4, with the most significant degradation shown in FIG. 4. Notably, rays of all wavelengths strike all parts of the grating. Consequently, for a particular wavelength, rays from all parts of the grating will cluster around the chief ray for that wavelength at the focal surface. Typically, the grating subtends an angle of about 33° from a point on the focal surface. Consequently, the rays approach the focal surface at the angle of the chief ray ±17°.
To better understand the significance of the conditioner exit surface curvature, it is helpful to consider three different conditioner exit surface radii: 1) flat; 2) equal to the radius of the grating; and 3) equal to half the radius of the grating (the radius of the Rowland circle).
In the case of a flat exit surface, for wavelengths lying on either side of the exit surface vertex, increasing the distance from the vertex both increases the angle of incidence of the chief ray and refracts the rays away from the vertex normal. For the surface radius equal to the grating radius, the chief ray is normal to the exit surface for all wavelengths. For the case of half the grating radius (the Rowland circle), the angle of incidence of chief rays for wavelengths lying on either side of the exit surface vertex increases with distance from the vertex, but the rays are refracted increasingly toward the vertex normal. For thermal detectors, a gap is required between the exit surface of the conditioner and the detector elements. In vacuum the required gap is much smaller than in air. Increasing the size of the gap increases detector responsivity but requires shortening the distance from the exit surface to be closer to the grating in order to maintain focus on the detector. A compromise must be made among a) focus on the detector, b) intensity getting to the detector, and c) the detector responsivity. If curved detector arrays were presently available, the choice would be a radius between that of cases 2 and 3. With a flat detector array, for example, the useful length of the array is limited to about 25% of the grating radius. At that length the sag is 0.016 of the grating radius. A flat exit surface on the conditioner gives the chief ray angles of incidence on the exit surface of ±14°. For longer arrays, it is very difficult to find a useful compromise among focus, intensity and responsivity.
Prior art reflective concave gratings, including the grating disclosed in U.S. Pat. No. 5,856,870, typically have a fixed blaze angle which is optimized for the center wavelength of a spectral region of interest. The blaze angle is chosen to maximize the diffraction efficiency for designated wavelengths. For a fixed blaze angle chosen to maximize the efficiency near the grating vertex, the efficiency falls off away from the vertex as the surface normal deviates increasingly from the vertex normal. A blaze angle deviating only 5° from the angle for maximum efficiency may drop the efficiency by half. The other half is directed into other diffraction orders. With a large numerical aperture system the grating may have an arc length of 34° or ±17° from the vertex. To keep the grating efficiency above 70% of peak efficiency, the blaze angle is preferably less than 3° from the blaze angle for highest efficiency.
A configuration of the conditioner having the entrance slit and the exit surface near or on the focal surface with the vertex normal of the exit surface vertex lying on the same line as the vertex normal of the grating minimizes the refracted angles of the exiting light.